Reservoir compaction and associated bedding plane slip and overburden shear can induce damage to wells. Casing damage leads to loss of pressure, impairment, pinching of production tubing, or an inability to lower workover tools.
Usually, casing impairment arises through shear owing to displacement of the rock strata along bedding planes or along steeply inclined fault planes. There are certain indicators pointing towards reservoirs which are most likely to suffer casing damage due to reservoir compaction.
In a stress-sensitive field, caliper logs and VDL recording in large number of wells with respect to time, may provide preliminary physical evidence of this phenomena and casing deformation pattern.
At the farthest end of the spectrum, it may be required to carry out 3D geomechanical simulation in conjunction with reservoir simulation and independent use of only one of these simulators may not give the complete picture. This would help in deciding on placement of wells, besides mitigation options to be considered during drilling of wells.
In an Indian onshore field, casing failure in seven wells (out of 41 drilled) has been reported. This high failure rate, and that too within a short span of few years, necessitated a closer look at reservoir behaviour. The paper describes the qualitative analysis carried out which leads to the conclusion that there is a likelihood of this field being stress-sensitive.
In petroleum industry, hydrocarbon production from unconsolidated reservoir induces geo-hazards like reservoir compaction and surface subsidence which may result in well failure, even platform sinking (in offshore environment). A classic example is, subsidence in Ekofisk Chalk field - the jack-up remediation for this field eventually cost around US $ 3 billion(1). Geomechanics addresses problems such as subsidence due to compaction of a reservoir, casing failure due to sand production and reservoir compaction. Casing impairment through shear takes place whenever large scale stress changes are induced in weak stratified sediments in a stress-sensitive reservoir. Compaction cases and thermal stimulation almost always generate a number of casing shear incidence. This has been documented in some of the well known case histories which include, Ekofisk and Valhall fields in North sea; Wilmington Oil Field, California; Groningen Gas Field, Netherlands; Boliver Coastal Oil Fields, Venezuela; Po Delta Gas Field, Italy; Belridge and Lost Hill Oil Field, California (1–15).
Several reservoir conditions may make a reservoir stress-sensitive. In the present case study, geomechanical considerations assume significance in view of the fact that a good number of wells are planned to be drilled under next phase development.
The phenomenon of well casing impairment/damage has considerable impact on field productivity and cost of production. These include:
Well casing damage leads to pinching of production tubing making it difficult or sometimes making it impossible for workover tools to run through. Many corrective actions which a well demands (e.g. gravel pack) may not be possible owing to casing impairment; this may consequently result in frequent well interventions (for bottom clearing) and thus increase the cost of production.
